Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the
deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area,
the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling
results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has
been active since ~0.78–0.436 Ma and accommodated ~13 to 34% of the estimated plate convergence
during that time. The remainder has likely been distributed among out-of-sequence thrusts further
landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the
Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore,
pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be
related to coarser material with higher permeability or more complex patterns of faulting that could
potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well-drained,
sand-rich material under the frontal thrust could have increased fault strength and helped to
maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from
seismic reflection data. Associated décollement propagation into weaker sediments at depth may help
explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor
bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest
removal of overlying material. Overall, results provide insight into the complex interactions between
incoming materials, deformation, and fluids in the frontal thrust region.